You are confusing two different things here.
The doctrine of "co-redemptrix" is currently a theological theory. This doctrine has neither been condemned, nor affirmed. Catholics are free, currently, to reject or believe the doctrine as they wish. Pope Francis and Pope Benedict have not spoken in favor of the doctrine, however. They both are against formally defining the doctrine.
In reading their statements, however, my personal opinion is that they are condemning caricatures of the doctrine--that is--what they are actually condemning is not the doctrine itself, but misstatements of the doctrine. In other words--I am not sure they actually know what is and is not being asserted with regard to the doctrine. That is just my opinion. Either way, currently, Catholics are free to accept or reject the doctrine as they see fit. The doctrine has the same status as the doctrine of Limbo. Though the trend of the Magesterium with regard to Limbo--has been to move away from the doctrine.
"Gate of Heaven" is a title of Mary that is completely true. Jesus joined heaven and earth in himself---and he came to earth to do this through Mary. Put simply: Jesus is heaven come to earth---but he did so by passing through Mary. So Mary is the Gate of Heaven. If she is not the Gate of Heaven, I do not know what is. This by the way is part of the doctrine of co-redemptrix. Mary is the second Eve. Her "yes" paves the way for the Second Adam. Just as the fall of humanity happened with a woman and a man, so also the redemption of humanity happens in an analogous manner. When Paul refers to Jesus as the Second Adam, implicit in that is Mary is the second Eve. If there is a Second Adam, there has to be a second Eve.
In the Old Testament, the Ark was the Gate of Heaven. It was the means by which God was present to his people. The Ark bore the presence of God on the Mercy Seat.
Now you are conflating mediation and the title "Gate of Heaven."
1) When Scripture talks about there being one mediator between God and mankind, Scripture means between God the Father and mankind. Jesus is the mediator between God the Father and mankind. And of course this is affirmed in passages all over the New Testament. Jesus is the sole way to the Father. No one comes to the Father but by Him.
2) With, through, and in Christ, however, we are all mediators. We can all boldly approach the throne of Christ. Thus, the meditation being talked about with regard to Mary is her mediation not before the throne of the Father, but before the throne of Christ. As we are all mediators before the throne of Christ, as we can all boldly approach the throne of Grace, it is difficult to understand why Protestants have such difficulty with the idea of Mary as mediator.